Previous work from this laboratory has provided conclusive evidence that changes in the rate of protein degradation constitute the single most important variable in the regulation of cellular protein content during liver growth. The aim of our current research is to contribute to a better understanding of the mechanism(s) involved in the degradation of cell proteins and its regulation. It includes: (a) a study on the pathways of degradation of abnormal proteins in mouse reticulocytes, taking advantage of our recent discovery that bestatin (an aminopeptidase inhibitor), taking advantage of our recent discovery that bestatin (an aminopeptidase inhibitor) produces accumulation of small peptide intermediates in this process, and (b) a study of the effects of amono acids on the degradation of different classes of proteins by isolated mouse hepatocytes, and the participation of tRNA in the response of these cells to amino acids starvation.